Accurate ${}^{238}\mathrm{U}$($n, 2n$)${}^{237}\mathrm{U}$ reaction cross-section measurements from 6.5 to 14.8 MeV

The cross section for the ${}^{238}\mathrm{U}(n,2n){}^{237}\mathrm{U}$ reaction has been measured in the incident neutron energy range from 6.5 to 14.8 MeV in small energy steps using an activation technique. Monoenergetic neutron beams were produced via the ${}^2\mathrm{H}(d,n){}^3\mathrm{He}$ and ${}^3\mathrm{H}(d,n){}^4\mathrm{He}$ reactions. ${}^{238}\mathrm{U}$ targets were activated along with Au and Al monitor foils to determine the incident neutron flux. The activity of the reaction products was measured in TUNL's low-background counting facility using high-resolution $\gamma$-ray spectroscopy. The results are compared with previous measurements and latest data evaluations. Statistical-model calculations, based on the Hauser-Feshbach formalism, have been carried out using the ${\mathrm{CoH}}_3$ code and are compared with the experimental results. The present self-consistent and high-quality data are important for stockpile stewardship and nuclear forensic purposes as well as for the design and operation of fast reactors.

Figure 1

Calculated energy distribution of neutrons hitting the ${}^{238}\mathrm{U}$ target at the 12.5-MeV settings. The spread in neutron energies is primarily caused by kinematic effects due to the extended geometry of the deuterium gas cell and the target sample.

Figure 2

Measured absolute efficiency for a HPGe Clover detector (sum of all four crystals) using a mixed nuclide $\gamma$-ray source. The solid curve is the fit to the data values (see text for details). A large error associated with the 165.8 keV $\gamma$ ray is due to the error in the measured intensity data ($\sim 10\%$ [28]).

Figure 3

$\gamma$-ray spectra measured before (a) and after activation [(b) and (c)] of the ${}^{238}\mathrm{U}$ target with $E_n=10.5$ MeV neutrons. The decay of ${}^{237}\mathrm{U}$ associated with the $\gamma$-ray line at 208.01 keV is shown at two different decay times. The activation conditions (irradiation, measurement, and decay times) are also given.

Figure 4

$\gamma$-ray spectra measured after activation of the ${}^{27}\mathrm{Al}$ (a) and ${}^{197}\mathrm{Au}$ (b) monitor foils with $E_n=6.5$ and 13.6 MeV, respectively, are shown at two different times (top and bottom). The characteristic $\gamma$-ray lines are indicated. The vertical dash lines are for guiding purposes only. The activation conditions (irradiation, measurement, and decay times) are also given.

Figure 5

Determination of half-lives of the activation products ${}^{237}\mathrm{U}$ (a and b), ${}^{196}\mathrm{Au}$ (c), and ${}^{24}\mathrm{Na}$ (d). The $\gamma$-ray transitions used for these nuclei are listed in Table 2. Note the logarithmic (base $e$) scales.

Figure 6

Present experimental cross-section data (down-ward pointing triangle) for the ${}^{238}\mathrm{U}(n,2n){}^{237}\mathrm{U}$ reaction are plotted as a function of incident neutron energy and shown in comparison to previous data from the literature as well as to the latest evaluations.

Figure 7

Comparison of the present data to the data taken from the latest evaluations.

Figure 8

Hauser-Feshbach model based theoretical calculation is compared to the experimental cross-section data for the ${}^{238}\mathrm{U}(n,2n){}^{237}\mathrm{U}$ reaction.